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Best engine oil to use in a 4.0?

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Getting ready to change my oil, and heard these straight 6's don't like synthetic oil, causes them to leak. Don't want to mess anything up, so I thought I would ask, for a Texas Jeep, what is a good oil to put into a TJ with 112K on it?
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Getting ready to change my oil, and heard these straight 6's don't like synthetic oil, causes them to leak. Don't want to mess anything up, so I thought I would ask, for a Texas Jeep, what is a good oil to put into a TJ with 112K on it?
That is like asking what is the best kind of air to breath. Any oil is good. That being said, I have three JEEPS with the straight six and use nothing but synthetic for the past 30 years. No leaks no problems. This is a can of worms where everyone will say something different.
 

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My oil of choice is Shell Rotella T/Chevron Delo/Mobil Delvac 15w-40.
 

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Synthetic oil doesn't cause anything to leak.It finds spaces that regular oil can't get out of and does.:( Valvoline and Castrol GTX are good.
 

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10w30 Quaker State High Mileage here. I know guys that have had this motor for years, and only bought what was on sale, never had a problem. Know guys that use Mobil 1 and have all kids of problems. Then again, I also know the complete opposite of what I just posted. So, to each their own. Each Jeep is completely different.
 

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I have used Mobil 1 in every vehicle I've ever owned with great results. I would also suggest amsoil and royal purple as well, but those are expensive and not really worth it for engine oil IMO. I change my oil every 5-6k miles
 

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Valvoline 10w30 is what I look for...
 

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Discussion Starter · #15 ·
Does it do any harm switching back to regular [new] oil, from, say unknown or possibly synthetic oil? I really have no idea what is in it right now, and want to switch to a known good oil of proper weight for Texas.
 

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What about using a semi-synthetic oil? Would that be more likely to find places to leak from than a standard oil would?
 

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Royal Purple 10w30 synthetic in my engine.

AMSOIL full synthetic in my diffs, transfer case and transmission.


No leaks, and i got 5k miles between intervals. oil is always full after the 5k. just a bit dirty.
 

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Does it do any harm switching back to regular [new] oil, from, say unknown or possibly synthetic oil? I really have no idea what is in it right now, and want to switch to a known good oil of proper weight for Texas.
No problems. Just make sure to drain as much as possible. Maybe walk away for an hour or so and let it drip in the catch pan. But it really won't make that much of a difference. Just buy a quality oil in the correct weight (10W30) and change it every 3k...:cheers2:
 
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